The present invention relates to a catalyst and process for the preparation of hydrocarbons from synthesis gas.
The preparation of hydrocarbons from a gaseous mixture comprising carbon monoxide and hydrogen (synthesis gas) by contacting the mixture with a catalyst at elevated temperature and pressure is well known in the art and is commonly referred to as Fischer-Tropsch synthesis.
Catalysts that may be suitably used in a Fischer-Tropsch synthesis process typically contain one or more catalytically active metals from Groups 8 to 10 of the Periodic Table of the Elements. In particular, iron, nickel, cobalt and ruthenium are well known catalytically active metals for such catalyst and may be optionally combined with one or more metal oxides and/or metals as promoters. Cobalt has been found to be the most suitable for catalysing a process in which synthesis gas is converted to primarily paraffinic hydrocarbons containing 5 or more carbon atoms. In other words, the C5+ selectivity of the catalyst is high.
Similar catalyst compositions are also known in other fields including JP-A-404122454 which describes an exhaust gas purification catalyst comprising an active platinum group element such as ruthenium, rubidium, palladium and platinum or a metal oxide such as chromium, manganese, cobalt and nickel on an alumina, silica, titania, silica-titania or alumina-titania carrier. Catalysts of the invention are fitted in an exhaust gas purification catalytic converter and may be used in controlling emissions from gasoline engines.
U.S. Pat. No. 5,134,109 provides a catalyst for the steam reforming of hydrocarbons, which comprises at least one metal from rhodium and ruthenium and at least one metal from cobalt and manganese deposited on a carrier which is preferably zirconia or stabilised zirconia.
JP-A-60064631 discloses a catalyst comprising an iron group metal such as cobalt and iron, a platinum group metal such as ruthenium, rhodium, palladium, platinum and iridium, and manganese oxide, supported on a carrier comprising titanium oxide. JP-A-60064631 further discloses a method for the production of high calorie gas containing hydrocarbons of 1-4 carbons for use as fuels, from low calorie gas containing a mixture of hydrogen, carbon monoxide and optionally carbon dioxide.
JP-A-60064631 is primarily concerned with a method for the production of methane and C2-4 hydrocarbons and does not concern itself in any way with increasing % C5+ selectivity during the conversion of low calorie gas. Indeed, it can seen from Example 2 therein, which is the only example of conversion of a simple CO/H2 mixture, that the treatment of a mixture of 3 parts H2 and 1 part CO in the presence of a catalyst composition comprising 10%. Co, 6% Mn2O3 and 2% Ru on a titanium carrier, results in 74.6% CH4, 7.3% C2H6, 5.5% C3H8, 2.6% C4H10 and 10.0% CO2 (by % volume), i.e. the presence of C5+ hydrocarbons was not detected. This conversion was effected at 320xc2x0 C., and although the broadest temperature range disclosed for the process is 150 to 400xc2x0 C., it is stated that the preferred range is 260 to 350xc2x0 C.
Although, U.S. Pat. No. 4,568,663 describes a rhenium-promoted cobalt catalyst on an inorganic oxide support which is preferably titania, which catalyst may be employed in production of hydrocarbons by both FT synthesis and the conversion of methanol, as being highly active, this disclosure is discussed in column 2, lines 19 to 35, of U.S. Pat. No. 4,857,559, and contrasted with the corresponding alumina-supported catalyst, which has significantly higher activity.
Much research effort has been directed to finding catalysts having a higher C5+ selectivity than known catalysts at the same or higher activity.
U.S. Pat. No. 4,857,559 concerns the addition of rhenium to cobalt on a number of common supports including alumina, silica, titania, chromia, magnesia and zirconia and a process for the FT synthesis of hydrocarbons using said catalyst. However, it is recognised therein (e.g. column 4, lines 54 to 59 and column 15, lines 51-55) that whilst supports other than alumina may be used, such supports produce catalysts with much lower activities. It is found in U.S. Pat. No. 4,857,559 that the hydrocarbon yield obtained by the addition of rhenium to alumina-supported cobalt catalyst is greater than the corresponding titania-supported catalyst. In particular, the FT conversion of synthesis gas into hydrocarbons show lower % CH4 selectivity, higher % CO conversion and higher C2+ selectivity for rhenium-promoted cobalt catalysts on alumina, than for similar catalysts on titania (Table 1).
Based on the above disclosure, the person skilled in the art would clearly deduce that TiO2 should be avoided as catalyst carrier for rhenium/cobalt combinations in favour of Al2O3.
Fischer-Tropsch synthesis of hydrocarbons produces a number of by-products such as carbon dioxide, water and gaseous C1-4 hydrocarbons.
As well as improving % CO conversion, it is of prime importance to be able to adjust the product slate in any given Fischer-Tropsch reaction, to satisfy individual requirements such as increased % C5+ selectivity and reduced CH4 and CO2 production.
It is highly desirable to reduce the amount of carbon dioxide evolved during Fischer-Tropsch synthesis of hydrocarbons for both economic and environmental reasons. It is particularly desirable to restrict the level of carbon dioxide by-product in such process to less than 2% v/v, preferably less than 1% v/v.
Of prime importance is that any methodologies employed for a reduction in carbon dioxide selectivity in Fischer-Tropsch synthesis, do not cause a concomitant reduction in C5+ hydrocarbon selectivity.
It can be seen from Table 1, that whilst the addition rhenium to a cobalt catalyst on titania gives a modest increase in activity from 11% carbon monoxide conversion to 17% carbon monoxide conversion, the C2+ selectivity is reduced and the CO2 selectivity is equal or increased compared to the corresponding unpromoted catalyst.
WO-A-97/00231 relates to a catalyst comprising cobalt and manganese and/or vanadium supported on a carrier wherein the cobalt: (manganese+vanadium) atomic ratio is at least 12:1.
Said catalyst exhibits higher C5+ selectivity and higher activity when used in the Fischer-Tropsch synthesis of hydrocarbons, compared to catalysts containing cobalt only, or containing a relatively higher amount of manganese and/or vanadium. Preferred carriers include titania, zirconia and mixtures thereof.
It is highly desirable not only to increase further the C5+ selectivity of such cobalt manganese catalysts, but also to reduce their carbon dioxide selectivity.
It has now been surprisingly found that the addition of small quantities of rhenium and/or ruthenium to cobalt-manganese catalyst compositions can not only cause reductions in carbon dioxide selectivity, but can also have dramatic effects on the product slate obtained from FT hydrocarbon synthesis reactions.
Accordingly, the present invention provides a process for the production of mainly C5+ hydrocarbons comprising contacting carbon monoxide and hydrogen at a temperature in the range of from about 180xc2x0 C. to about 270xc2x0 C. and elevated pressure in the presence of a catalyst composition comprising cobalt, manganese and at least one of rhenium and/or ruthenium on a titania carrier.
According to another aspect, the present invention provides a catalyst composition comprising cobalt, manganese and rhenium on a titania carrier.
By xe2x80x9cmainlyxe2x80x9d in the present invention is meant an amount greater than about 80 wt. % based on the paraffinic hydrocarbon and carbon dioxide distribution.
By xe2x80x9cproduct slatexe2x80x9d in the present invention is meant the overall product distribution resulting from Fischer-Tropsch synthesis of hydrocarbons, i.e. the relative amounts of C1-4 hydrocarbons, C5+ hydrocarbons, water and carbon dioxide present in the product mixture.
The rutile:anatase weight ratio in the titania carrier is not limited in the present invention, however said ratio may conveniently be less than about 2:3, as determined by ASTM D 3720-78.
The titania carrier may be prepared by any method known in the art, however it is particularly preferred that the titania carrier is prepared in the absence of sulphur-containing compounds. An example of such a preparation method involves flame hydrolysis of titanium tetrachloride. It will be appreciated that the titania powder derived from such a preparation method may not be of the desired size and shape. Thus, usually a shaping step is required to prepare the carrier. Shaping techniques are well known to those skilled in the art and include pelletising, extrusion, spray-drying, and hot oil dropping methods.
Titania is available commercially and is well-known as a material for use in the preparation of catalysts or catalyst precursors.
As an alternative or in addition to titania, the mixture may comprise a titania precursor. Titania may be prepared by heating titanium hydroxide. As the heating progresses, titanium hydroxide is converted via a number of intermediate forms and the successive loss of a number of water molecules into titania. For the purpose of this specification, the term xe2x80x9ctitania precursorxe2x80x9d is to be taken as a reference to titanium hydroxide or any of the aforementioned intermediate forms.
Catalysts that may be used in the process of present invention preferably contain from about 5 wt. % to about 30 wt. % of Co based on the total weight of the catalyst composition, more preferably from about 10 wt. % to about 25 wt. % of Co based on the total weight of the catalyst composition, and most preferably about 15 wt. % to about 25 wt. % of Co based on the total weight of the catalyst composition.
Catalysts that may be used in the process of present invention preferably contain from about 0.01 wt. % to about 5 wt. % of Mn based on the total weight of the catalyst composition, more preferably from about 0.01 wt. % to about 1.5 wt. % of Mn based on the total weight of the catalyst composition.
Catalysts that may be used in the process of present invention preferably contain from about 0.01 wt. % to about 5 wt. % each of rhenium and/or ruthenium based on the total weight of the catalyst composition, more preferably from about 0.01 wt. % to about 1 wt. % each of rhenium and/or ruthenium based on the total weight of the catalyst composition, and most preferably from about 0.01 wt. % to about 0.5 wt. % each of rhenium and/or ruthenium based on the total weight of the catalyst composition.
Catalysts that may be used in the process of the present invention may further comprise up to about 20 wt. % of a binder material such as alumina or silica based on the total weight of the catalyst composition, preferably up to about 10 wt. % of a binder material such as alumina or silica based on the total weight of the catalyst composition.
The pore size and volume of the catalyst composition prior to activation are not limited in the present invention. The pore volume may conveniently be in the range of from about 0.1 cm3/g to about 0.8 cm3/g, preferably in the range of from about 0.15 cm3/g to about 0.7 cm3/g, more preferably in the range of from about 0.2 cm3/g to about 0.5 cm3/g.
The catalyst of the present invention may be prepared by methods known to those skilled in the art, such as by precipitating the catalytically active compounds or precursors onto the carrier; spray-coating, kneading and/or impregnating the catalytically active compounds or precursors onto the carrier; and/or extruding one or more catalytically active compounds or precursors together with carrier material to prepare catalyst extrudates.
It will be appreciated by those skilled in the art that the most preferred method of preparation may vary, depending e.g. on the desired size of the catalyst particles. It belongs to the skill of the skilled person to select the most suitable method for a given set of circumstances and requirements.
Extrusion may be effected using any conventional, commercially available extruder. In particular, a screw-type extruding machine may be used to force the mixture through the orifices in a suitable dieplate to yield extrudates of the desired form. The strands formed upon extrusion may be cut to the desired length.
After extrusion, the extrudates are dried. Drying may be effected at an elevated temperature, preferably up to about 500xc2x0 C., more preferably up to about 300xc2x0 C. The period for drying is typically up to about 5 hours, more preferably from about 15 minutes to about 3 hours.
The extruded and dried catalyst composition may be calcined. Calcination is effected at elevated temperature, typically in the range of from about 200xc2x0 C. to about 900xc2x0 C., preferably at a temperature in the range of from about 400xc2x0 C. to about 750xc2x0 C., more preferably in the range of from about 500xc2x0 C. to about 650xc2x0 C. The duration of calcination treatment is conveniently from about 5 minutes to several hours, preferably from about 15 minutes to about 4 hours.
Conveniently, the calcination treatment is carried out in an oxygen-containing atmosphere, preferably air. It will be appreciated that, optionally, the drying step and the calcining step may be combined.
A preferred method of preparing the catalyst according to the present invention is by impregnating the catalytically active compounds or precursors onto a carrier. Thus, typically the carrier is impregnated with a solution of a cobalt compound, a solution of a rhenium and/or ruthenium compound and a solution of a manganese compound. Preferably, the carrier is impregnated simultaneously with the respective metal compounds. Thus, according to a preferred embodiment, the process for preparing the catalyst for use in the process of the present invention comprises co-impregnating the carrier with a solution of a cobalt compound, a solution of a rhenium and/or ruthenium compound and a solution of a manganese compound.
A further preferred method of preparing the catalyst according to the present invention is by mixing some of the catalytically active compounds or precursors with the carrier, followed by extruding the resulting mixture, followed by drying and calcining the extrudate, followed by impregnating with further catalytically active compounds or precursors to prepare catalyst extrudates for use in the process of the present invention.
Thus, typically the carrier is mixed with a cobalt compound and a manganese compound and water followed by extrusion of the resulting mixture, and after drying and calcining, followed by impregnation with a solution of a rhenium and/or ruthenium compound to prepare catalyst extrudates for use in the process of the present invention. Preferably, the carrier is mixed simultaneously with the cobalt and manganese compounds.
Thus, according to a preferred embodiment, the process for preparing the catalyst for use in the process of the present invention comprises co-extruding the carrier with a cobalt compound and a manganese compound, followed by impregnating the extrudate with a solution of a rhenium and/or ruthenium compound.
Examples of suitable cobalt compounds that may be used in the preparation of said catalyst include one or more of cobalt hydroxide, cobalt oxide, cobalt carbonyl, halides such as cobalt chloride (hexahydrate or anhydrous), inorganic acid salts such as cobalt sulphate, cobalt nitrate or cobalt carbonate, and organic acid salts such as cobalt acetate and cobalt formate. Preferred cobalt compounds include cobalt hydroxide, cobalt carbonate and cobalt nitrate.
Examples of suitable rhenium compounds that may be used in the preparation of said catalyst include one or more of rhenium oxide, rhenium chloride, rhenium carbonyl, ammonium perrhenate and perrhenic acid. The preferred rhenium compound is ammonium perrhenate.
Examples of suitable ruthenium compounds that may be used in the preparation of said catalyst include one or more of a ruthenium halide such as ruthenium chloride or ruthenium iodide, a ruthenic halide or salt thereof, for example ruthenic chloride, ammonium ruthenic chloride, sodium ruthenic chloride, potassium ruthenic chloride, a ruthenium oxide such as ruthenium di or tetraoxide, a ruthenic acid salt such as potassium ruthenate or sodium ruthenate, an organic ruthenium compound such as ruthenium carbonyl, ruthenium nitrosyl nitrate. The preferred ruthenium compound is ruthenium nitrosyl nitrate.
Examples of suitable manganese salts that may be used in the preparation of said catalyst include one or more of manganese hydroxide, manganese oxide, halides such as manganese chloride, inorganic acid salts such as manganese sulphate, manganese nitrate or manganese carbonate, and organic acid salts such as manganese acetate and manganese formate. Preferred manganese compounds include manganese hydroxide, manganese nitrate and manganese acetate.
The impregnation treatment is typically followed by drying and, optionally, calcining. Drying is typically carried out at a temperature in the range of from about 50xc2x0 C. to about 300xc2x0 C. for up to about 24 hours, preferably from about 1 to about 4 hours.
Calcination is typically carried out at a temperature in the range of from about 200xc2x0 C. to about 900xc2x0 C., preferably, in the range of from about 250xc2x0 C. to about 700xc2x0 C. The duration of the calcination treatment is typically from about 0.5 to about 24 hours, preferably from about 1 to about 4 hours. Suitably, the calcination treatment will normally be higher than the average temperature during the drying treatment.
When in use, the catalyst for the process of the present invention may contain at least part of the cobalt in its metallic form.
Therefore, it is normally advantageous to activate the catalyst prior to use by a reduction treatment, in the presence of hydrogen at elevated temperature. Typically, the reduction treatment involves treating the catalyst at a temperature in the range of from about 100xc2x0 C. to about 450xc2x0 C. for about 1 to about 48 hours at elevated pressure, typically from about 0.1 MPa to about 20.0 MPa (1 to 200 bar abs.). Pure hydrogen may be used in the reduction treatment, but it is usually preferred to apply a mixture of hydrogen and an inert gas, such as nitrogen. The relative amount of hydrogen present in the mixture may range from between 0% to about 100% by volume.
According to one preferred embodiment, the catalyst is brought to the desired temperature and pressure level in a nitrogen gas atmosphere and subsequently, the catalyst is contacted with a gas mixture containing only a small amount of hydrogen gas, the rest being nitrogen gas. During the reduction treatment, the relative amount of hydrogen gas in the gas mixture is gradually increased up to about 50% or even about 100% by volume.
If possible, it is preferred to activate the catalyst in situ i.e. inside the reactor. WO-A-97/17137 describes an in situ catalyst activation process which comprises contacting the catalyst in the presence of hydrocarbon liquid with a hydrogen-containing gas at a hydrogen partial pressure of at least about 1.5 MPa (15 bar abs.), preferably at least about 2.0 MPa (20 bar abs.), more preferably at least about 3.0 MPa (30 bar abs.). Typically, in this process the hydrogen partial pressure is at most about 20 MPa (200 bar abs.).
It is advantageous to rejuvenate spent catalyst, i.e. catalyst that has lost at least part of the initial activity of an activated fresh catalyst, by subjecting it to a hydrogen strip or an ROR treatment. Conveniently, the ROR treatment involves the steps, in sequence, of reduction with a hydrogen-containing gas, oxidation with an oxygen-containing gas, and reduction with a hydrogen-containing gas.
The process of the present invention is preferably carried out at a temperature in the range of from about 200xc2x0 C. to about 250xc2x0 C. The pressure is typically in the range of from about 0.5 MPa to about 15.0 MPa (5 to 150 bar abs.), preferably in the range of from about 1.0 Mpa to about 8.0 MPa (10 to 80 bar abs.), in particular from about 1.0 Mpa to about 6.0 MPa (10 to 60 bar abs.).
Hydrogen and carbon monoxide (synthesis gas) may be conveniently fed to the process at a molar ratio in the range of from 1 to 2.5.
The gas hourly space velocity (GHSV) of the synthesis gas in the process of the present invention may vary within wide ranges and is typically in the range of from about 400 to about 10000 N1 lxe2x88x921hxe2x88x921, for example from about 400 to about 4000 N1 l31 1hxe2x88x921. The term GHSV is well known in the art, and relates to the volume of synthesis gas in N1, i.e. litres at standard temperature and pressure (STP) conditions (0xc2x0 C. and 1 bar abs.), which is contacted in one hour with one litre of catalyst particles, i.e. excluding interparticular void spaces for slurry reactions. In the case of a fixed catalyst bed, the GHSV may also be expressed as per litre of catalyst bed, i.e. including interparticular void space.
The process of the present invention for the preparation of hydrocarbons may be conducted using a variety of reactor types and reaction regimes, for example a fixed bed regime, a slurry phase regime or an ebullating bed regime. It will be appreciated that the size of the catalyst particles may vary depending on the reaction regime they are intended for. It belongs to the skill of the skilled person to select the most appropriate catalyst particle size for a given reaction regime.
Further, it will be understood that the skilled person is capable of selecting the most appropriate conditions for a specific reactor configuration and reaction regime. For example, the preferred gas hourly space velocity may depend upon the type of reaction regime that is being applied. Thus, if it is desired to operate the hydrocarbon synthesis process with a fixed bed regime, preferably the gas hourly space velocity is chosen in the range of from about 500 to about 2500 N1 lxe2x88x921hxe2x88x921. If it is desired to operate the hydrocarbon synthesis process with a slurry phase regime, preferably the gas hourly space velocity is chosen in the range of from about 1500 to about 7500 N1 lxe2x88x921hxe2x88x921.
The present invention is illustrated by the following Examples, which should not be regarded as limiting the scope of the invention in any way.